1. Technical Field
The present invention relates to a vehicle power steering system, and more specifically to a hydraulic vehicle power steering system in which the resistance to actuation of a power steering control valve increases with increasing vehicle speed.
2. Description of Related Art
A vehicle power steering system including a rotary control valve is shown in U.S. Pat. No. 5,293,954. The valve has an inner valve member that is coaxial with and rotatable relative to an outer valve member or sleeve. To effect actuation of the power steering motor to turn steerable vehicle wheels, the inner valve member is rotated relative to the outer valve member. A fluid pressure reaction chamber is provided to regulate the torque required to rotate the inner valve member relative to the outer valve member. The fluid pressure in the reaction chamber increases as vehicle speed increases to increase the resistance felt by an operator of the vehicle to rotation of the inner valve member relative to the outer valve member.
The valve sleeve is assembled into the control valve by sliding it axially until a hitch pin, press fitted in a pinion, moves into an axially extending slot in the end of the valve sleeve. The engagement of the hitch pin in the slot couples the valve sleeve for rotation with the pinion in a follow-up manner.
Changes in the fluid pressure in the reaction chamber affect the forces acting on the valve sleeve. These forces can tend to urge the valve sleeve to move axially in its housing, relative to the hitch pin and the pinion. Upon such movement, seals that are disposed between the valve sleeve and the housing can be forced out of their grooves and into the annular space between the valve sleeve and the housing. This can adversely affect the seals.
The present invention is a fluid power assist rack and pinion steering system for a vehicle having steerable wheels. The steering system comprises a rack connected with steering linkage of the vehicle for, upon movement of the rack, moving the steering linkage to effect turning of the steerable wheels. A rotatable pinion is in meshing engagement with the rack. The steering system also includes a control valve comprising a valve core rotatable relative to a valve sleeve. A first part connects the valve sleeve for rotation with the pinion. A mechanism resists relative rotation between the valve core and the valve sleeve as vehicle speed increases by placing axial force on the valve sleeve. A second part on the valve sleeve resists axial movement of the valve sleeve relative to the valve core.